A passive optical network (PON) is an access network technology that connects a central office entity called “Optical Line Terminal OLT” with the customer premises equipment called “Optical Network Unit ONU” over a maximum distance of several 10 kms. The devices are connected by optical fibers that are deployed in a tree structure. PONs are currently considered as a candidate for access network sharing as the optical fiber offers high bandwidth and is already deployed to a large number of customers.
(Access) network sharing is a paradigm that allows different operators, e.g., a mobile and a fixed operator to use the ‘last mile’ of the network conjointly. While the fixed operator uses this access part to directly attach its customers, the mobile operator uses the access to connect base stations. In access network sharing, the two networks will run on the same hardware (i.e. devices and physical links), but will be logically separated.
Radio over fiber (RoF) is a technology widely used in the design and the deployment of base stations, in which a baseband processing unit (BBU) is connected via an optical link to the radio equipment and via this to an antenna. The BBU receives the signals from the core network of a mobile operator, e.g. in case of a LTE network the evolved packet core EPC S1 signals and converts them to RoF signals, these are transmitted via the optical link to the radio equipment/antenna unit (which will be called remote radio head in the following sections) where they are modulated on the wireless channel.
If there is provided one BBU for each remote radio head, the hardware costs are significant. It is therefore desirable to use an approach where the baseband processing resources are “pooled” such that one BBU serves several remote radio heads. It is even further preferable if the network over which the RoF signals are transmitted is shared with a fixed network operator.
Such scenarios will now be explained in somewhat more detail in the following.
Radio-over-fiber is the digitized or analogue transmission of the radio layer signals via an optical fiber. Digital radio over fiber requires large amounts of bandwidth, between 10-30 times the bytes of the S1 traffic are necessary. A typical site with 3 cells with 1 Gbps S1 traffic each (for LTE-A) thus can require 60-70 Gbps RoF capacity. It is difficult to fit this high amount of bandwidth into a (shared) PON system. The presently most advanced TDM PON systems on the market today support 10 Gbps, shared between all users of the PON. WDM PON systems might support higher bandwidths, but whether 60-70 Gbps for a single user can be supported in the near future is questionable.
One alternative is to multiplex digital RoF signals via high-capacity, long-range technologies like CWDM or DWDM (coarse/dense wavelength division multiplexing) on the shared PON. However, both systems have their disadvantages: while CWDM is cheap, it uses a lot of the optical spectrum, which might be already occupied by the different FTTH systems. DWDM requires only a low amount of spectrum, however it is quite costly.
Another alternative is analogue RoF, however this only works via short distances as the transmission distance is limited by fiber distortions and non-linearities. At the same time, there is a trend in the fiber-to-the-home community to create access networks with long optical reach. This is advantageous as the amount of central offices can be reduced, thus saving CAPEX and OPEX.
When one looks at access network sharing and RoF, it becomes clear that this trend becomes problematic: it is the goal of FTTH-operators to cover larger and larger areas from a central location for cost saving reasons; however a mobile operator can only cover a part of the area with a centralized baseband pool and analogue RoF. Thus, the coverage area is not the same, and the mobile operator has to resort to (at least in the peripheral of the network, where a transmission of RoF signals from the central office is not possible) deploy conventional base station technology, without the benefit of baseband pooling.
As mentioned above, high-capacity long-range digital technologies also have their disadvantages with respect to spectrum usage and/or cost. Furthermore, (digital) RoF transmission distance is also limited from an unexpected direction, the radio layer: certain wireless protocols require a reaction within a strict time limit. A wireless signal that requires such a reaction initially arrives at the antenna, is then transmitted via RoF to the baseband processing where it is decoded and such a reaction is calculated. The reaction is transmitted back to the antenna, again via RoF, and then sent out wirelessly. In this processing and transmission chain, each kilometer of fiber over which the RoF signal is transported adds latency due to the pure signal propagation delay, while the overall time is limited by radio layer standards.
The three known approaches for baseband pooling and/or network sharing are illustrated in FIG. 1. The first one (shown on the left-hand side in FIG. 1) is the centralized baseband pooling at the PON's central office location with analogue or digital RoF. This has the above outlined limitations. The second solution (shown in the center of FIG. 1) is a distributed baseband processing, in which each unit connected to the passive optical network is a complete base station. Obviously, this approach does not allow realizing baseband pooling gains and only can achieve network sharing.
The last know solution (shown on the right-hand side in FIG. 1) is one in which the baseband processing unit is connected to one of the leaf nodes, and the remote radio heads of the other leaf nodes are connected to it via a dedicated network. Here, the length of the PON tree trunk is not problematic; however, the mobile operator has to deploy new fibers to attach the remote radio heads, thus not being able to realizing the network sharing gains.
It is therefore desirable to have a system where baseband pooling gains and network sharing gains can be achieved simultaneously, and even in cases where the PON covers a long distance. None of the existing solutions has such capabilities.